For the purposes of detection of combustion conditions or knocking, improvement of fuel economy, cleaning of exhaust gas, and so on, in an internal combustion engine, the pressure in a combustion chamber of the internal combustion engine has been heretofore detected by a pressure sensor built in a spark plug. The pressure sensor integrated spark plug used for such purposes will be summarized below. That is, a pressure sensing element constituted by a ring-like piezo ceramic element is fitted, together with a ring-like electrode for extracting an output, from outside into a base end position of an attaching screw portion formed in a metal shell of a spark plug. The pressure sensing element is caught by a collar-like sensor retention portion while the pressure sensing element as a whole is covered with a sensor housing from the outside. An output lead wire from the ring-like electrode is extracted rearward from the sensor housing. When the spark plug is attached to a plug hole of an internal combustion engine at the attaching screw portion, the pressure sensor is pressed onto an opening outer edge portion of the plug hole through the sensor housing. The combustion pressure is transmitted to the pressure sensor through the sensor housing. The pressure sensor outputs a voltage proportional to a detected pressure level through the output lead wire by the piezo-electric effect.
However, in the case of the structure in which the pressure sensing element has been built in the spark plug integrally and inseparably as described above, once the life time of the spark plug is expired, the pressure sensor portion together with the spark plug has to be wastefully exchanged for a new one even if the pressure sensor portion is in a working condition. Therefore, Japanese Patent Laid-Open No. 290853/1994 (FIGS. 7 and 8) discloses a configuration in which a pressure sensor is formed separately from a spark plug so that the pressure sensor can be attached to a newly exchanged spark plug and used continuously.
However, the pressure sensor configured thus has a structure in which a lead wire for extracting a sensor output projects sideways from the pressure sensing element and then led out on the rear end side of the spark plug. In such a structure, the pressure sensor is difficult to apply to recent engines in which space saving has been advanced. For example, in recent automobile gasoline engines, a mechanism in the periphery of a cylinder head to which a spark plug is attached is sophisticated. Particularly, in a structure in which a spark plug is attached to the bottom portion of a deep plug hole formed in a cylinder head, valve-system components and so on crowded around the plug hole bottom where a pressure sensing element is located. Thus, in not a few engines, no allowance is left for projecting the lead wire sideways. In addition, the lead wire extracted rearward occupies a certain space in the plug hole. However, in order to save the space in the engine room, there is a type in which a pencil-like ignition coil is installed in the plug hole. In this case, there may occur a problem that the lead wire becomes an obstacle to the installation of the ignition coil.